modify image pixels in forms

Hi I want to change a collared image to greyscale before I show it on the screen. Can some one please tell me why my code below is not working?

     private byte[] ConvertMediaFileToByteArray(MediaFile file)
        {
            using (var memoryStream = new MemoryStream())
            {
                file.GetStream().CopyTo(memoryStream);
                return memoryStream.ToArray();
            }
        }

        public void LoadAndConvertImage(MediaFile imgPath){
            byte[] imgSrc = ConvertMediaFileToByteArray(imagePath);
            ConvertToGrayRemoveNoise(ref cropedBytes);

             ImageSource imageSource = ImageSource.FromStream(() =>
            {
                var cropedImage = new MemoryStream(cropedBytes);
                file.Dispose();
                return cropedImage;
            });
            Image Image.Source = imageSource;

        }
        public void ConvertToGray(ref byte[] srcPixels)
        {
            try
            {
                for (int i = 0; i < srcPixels.Length; i += 4)
                {
                    double b = (double)srcPixels[i] / 255.0;
                    double g = (double)srcPixels[i + 1] / 255.0;
                    double r = (double)srcPixels[i + 2] / 255.0;

                    byte a = srcPixels[i + 3];
                    //convert to gray
                    double e = (0.299 * r + 0.587 * g + 0.114 * b) * 255;

                    byte f = Convert.ToByte(e);

                    srcPixels[i] = f;
                    srcPixels[i + 1] = f;
                    srcPixels[i + 2] = f;
                    srcPixels[i + 3] = a;

                }
            }
            catch (Exception error)
            {
                Debug.WriteLine(error.ToString());
            }
        }

I see two issues. Sometime the ConvertToGray function runs into the Catch with a pointer out of range. If this does not happen the initialisation of the image fails with "initWithData returned nil". I would like to work with the byte[] as several plugins with crop also use that as input and output.